This proposal will investigate the interactions between Borrelia burgdorferi and Ixodes scapularis ticks, in order to better understand Lyme disease pathogenesis. We will explore the hypothesis that the specific association between B. burgdorferi outer surface protein (Osp) A and the Tick Receptor for OspA (TROSPA) is required for the successful colonization of I. scapularis by the spirochete. We will focus on examining how OspA interacts with the arthropod vector because our published and preliminary studies demonstrate that OspA facilitates the adherence ofB. burgdorferi to the tick gut. The importance of OspA in the attachment of B. burgdorferi to the I. scapularis gut will be carefully examined. We will then identify, clone and characterize the TROSPA gene, express and purify TROSPA in recombinant form, and examine the interactions between OspA and TROSPA. In vitro and in vivo studies will then determine whether antibodies to TROSPA, or OspA peptides, can interfere with spirochete colonization of I. scapularis. Finally, we will assess the influence of B. burgdorferi on the expression of the TROSPA gene, exploring the hypothesis that B. burgdorferi upregulate TROSPA within the vector, in part to facilitate survival of the spirochete. The efforts on OspA and TROSPA will be the main focus of the grant proposal. It is also possible; however, that OspA is not the only B. burgdorferi ligand that interacts with I. scapuIaris, and we may consider other B. burgdorferi genes that are expressed within I. scapularis and that may associate with the vector as our research progresses. These studies will explore the intimate relationship between B. burgdorferi and I. scapularis - with an emphasis on OspA and TROSPA - leading to a greater appreciation of the B. burgdorferi lifecycle and new approaches to interfere with spirochete transmission.